This invention concerns the manufacture of color cathode ray tubes, and in particular the manufacture of color cathode ray tubes of the types which employ a tensed color selection electrode. A variety of color cathode ray tubes in the prior art have a phosphor screen, adjacent to which is a color selection electrode which controls the landing pattern of one or more electron beams as they are swept across the screen to trace out a television picture. Some of these tubes disclosed in the prior art employ a tensed color selection electrode -- others a non-tensed electrode. The most common type of color selection electrode in commercial use today is of the non-tensed type and is the so-called "shadow mask". A shadow mask is a color selection electrode in which color selection is accomplished, wholly or partially, by shadowing selected areas of the screen from the electron beams. The typical shadow mask in use today has an approximately spherical contour and is spring-supported on the envelope of the containing tube.
Of the prior art tubes which utilize a tensed color selection electrode, some are of the shadow mask variety. Others, confined largely to the laboratory as of this date, have one or more electrical potentials applied to the color selection electrode, which potential(s) differ from a potential applied to the screen so as to cause the electron beams to be focused and/or deflected from their free-flight path. Color selection in those latter prior art tubes is achieved either totally by the establishment of electrical fields, or partially by electrical fields and partially by mechanical shadowing of the screen.
This invention is applicable to the latter-described class of tubes in which the color selection electrode is tensed in at least one direction.
There exists in the marketplace today a color tube which utilizes a tensed shadow mask. The mask is understood to be placed under high tension by purely mechanical means. Specifically, a very heavy mask support frame is compressed prior to and during affixation of the mask to it. Upon release of the frame, restorative forces in the frame cause the mask to be placed under high residual tension. During normal tube operation, electron beam bombardment causes the mask to heat up and the mask tension to be reduced. An upper limit is placed on the intensity of the electron beams which may be used to bombard the screen without causing the mask to relax completely (buckle) and lose its color selection capability. The upper limit has been found to be below that required to produce color pictures of the same brightness as are produced in tubes having non-tensed shadow masks. For descriptions of examples of this type of tube, see U.S. Pat. No. 3,638,063 and "25-V Inch 114.degree. Trinitron Color Picture Tube and Associated New Developments, BTR," August, 1974, pp 103-200, by Yoshida et al. Also see U.S. Pat. Nos. 3,719,848 and 2,905,845, and "General Electric Post Acceleration Color Tube" by C. T. Lob, IRE Convention Record, Electron Devices and Receivers, 1956, pp. 114-117.
It is perhaps more common in the prior art of tensed electrode television tubes to find the electrode being installed by combining mechanical tensioning of the electrode with applied heat. Typically, in such prior art disclosures, a color selection electrode in the form of an apertured foil or a grid of wires is held in a holder which places the electrode under a predetermined amount of tension at room temperature. The electrode and the holder are then heated to a predetermined elevated temperature in an oven. By having selected a material for the electrode which has a coefficient of thermal expansion which is substantially greater than that of the holder, the electrode is caused to expand to a greater degree than the holder.
Typically the holder includes a part of the tube's envelope for ultimately securing the electrode and a temporary fixture for stretching the electrode before attachment of the electrode to the envelope. While at the predetermined elevated temperature, the electrode is sealed to the envelope -- e.g. between the faceplate and funnel thereof, or to an interior ledge on the envelope. Upon cool-down of the tube, the electrode contracts to a greater extent than does the envelope, causing the electrode to be placed under high tension.
This approach has the same drawback as the purely mechanical approach -- that is, an electrode installed by this method is incapable of absorbing a satisfactorily high electron beam current before it relaxes.
The latter-described prior art approach also suffers from the problem of maintaining the precision of the temporary fixture which goes into the oven with the tube. Examples of this latter approach to installing tensed color selection electrodes can be seen in U.S. Pat. Nos. 3,489,966, 2,842,696, 3,284,655, and 2,813,213, and in British Pat. No. 1,163,495.